Avaliação do sistema M-QAM OFDM sem e com tempo de guarda em um canal PLC com os ruídos Gaussiano e impulsivo e múltiplas reflexões

This article present an evaluation of the M-QAM OFDM system over PLC channel with effects of Gaussian and impulsive noises, and multiple reflections. Some tests with the PLC adaptor were made, and by analyzing the result, it was observed that the data transmission rate decreases in a nosy channel. Therefore, a model of combined “Bernoulli-Gaussian”, impulsive, and Gaussian noises was developed. It was analyzed the effect of this model in the designed equations for the calculus of the symbolic error probability for M-QAM modulation, with square and cross-nonsquare constellation. The results obtained for the simulation showed that the addition of impulsive noise increases the M-QAM symbolic error probability. A different equation design was proposed for the calculus of the symbolic error probability, however adding multiple carriers this time. The results for this simulation exposed that the MQAM OFDM noisy composition decreases the symbolic error probability. Another problem of the PLC channel is the multiple reflections effect, which causes a signal delay. In order to analyze this effect, two equations were proposed for the M-QAM OFDM system, with and without guard interval. By evaluating the obtained results using guard interval, it was detected a dropped of symbolic error probability. The guard interval reduced the intersymbolic effect.Tese (Doutorado)Este trabalho apresenta uma avaliação do sistema M-QAM OFDM em um canal PLC com os efeitos dos ruídos Gaussiano e impulsivo e das múltiplas reflexões. Alguns testes com o adaptador PLC foram realizados e analisando os resultados verificou-se a diminuição da taxa de transmissão de dados no canal com ruídos. Então, um modelo de ruído combinado “Bernoulli-Gaussiano”, impulsivo e Gaussiano, foi desenvolvido. Avaliou-se o efeito desse modelo nas equações desenvolvidas para o cálculo da probabilidade de erro de símbolo da modulação M-QAM com constelação quadrada e não quadrada cruzada. Os resultados obtidos das simulações mostraram que a adição do ruído impulsivo eleva a probabilidade de erro de símbolos da M-QAM. Um segundo equacionamento foi proposto para o cálculo da probabilidade de erro de símbolos e a este se acrescentou múltiplas portadoras. Nos resultados dessas simulações observou-se que a composição M-QAM OFDM diminuiu a probabilidade de erro de símbolos com ruído combinado. O outro problema no canal PLC é o efeito das múltiplas reflexões que ocasiona atraso do sinal. Para avaliar esse efeito foram propostos dois equacionamentos do sistema M-QAM OFDM sem e com intervalo de guarda. Analisando os resultados obtidos utilizando intervalo de guarda observou-se uma diminuição da probabilidade de erro de símbolo. O intervalo de guarda reduziu o efeito da interferência entre símbolos

Distributed Digital Radios for Land Mobile Radio Applications

The main objective of this dissertation is to develop the second generation of Distributed Digital Radio (DDR) technology. A DDR II modem provides an integrated voice/data service platform, higher data rates and better throughput performance as compared to a DDR I modem. In order to improve the physical layer performance of DDR modems an analytical framework is first developed to model the Bit Error Rate (BER) performance of Orthogonal Frequency Division Multiplexing over Frequency Modulation (OFDM/FM) systems. The use of OFDM provides a spectrally efficient method of transmitting data over LMR channels. However, the high Peak-to-Average (PAR) of OFDM signals results in either a low Signal-to-Noise Ratio (SNR) at FM receiver or a high non-linear distortion of baseband signal in the FM transmitter. This dissertation presents an analytical framework to highlight the impact of high PAR of OFDM signal on OFDM/FM systems. A novel technique for reduction of PAR of OFDM called Linear Scaling Technique (LST) is developed. The use of LST mitigates the signal distortion occurring in OFDM over FM systems. Another important factor which affects the throughput of LMR networks is the Push-to-Talk (PTT) delay. A PTT delay refers to the delay between the instant when a PTT switch on a conventional LMR radio is keyed/unkeyed and a response is observed at the radio output. It can be separated into a Receive-To-Transmit Switch Interval (RTSI) or a Transmit-To-Receive Switch Interval (TRSI). This dissertation presents the typical RTSI delay values, distributions and their impact on throughput performance of LMR networks. An analytical model is developed to highlight the asymmetric throughput problem and the unintentional denial of service (UDOS) occurring in heterogeneous LMR networks consisting of radios with different PTT delay profiles. This information will be useful in performance and capacity planning of LMR networks in future

New OFDM schemes based on orthogonal transforms for mobile communications systems :

PhD ThesisIn this thesis, two new orthogonal frequency division multiplexing (OFDM) systems are presented. The first scheme proposes a new OFDM system transceiver based on the C-transform, which is termed C-OFDM. Over multipath channels, the C-OFDM achieves 10 dB signal-to-noise ratio (SNR) gain at 10−4 bit-error-rate (BER), in comparison to the OFDM that based on the is discrete cosine transform (DCT-OFDM) and the conventional OFDM schemes. It also reduces the peak-to-average power ratio (PAPR) of the OFDM signal by about 1 dB and in some cases up to 3 dB. In the second scheme, a new fast, orthogonal X-transform is produced. The proposed X-transform is then used in a new OFDM named X-OFDM to greatly reduce the complexity, the PAPR and the BER. The proposed scheme achieves around 15 dB SNR gain in comparison to the conventional OFDM at 10−4 BER and reduces the average PAPR (over 105 OFDM symbol) by about 6 dB for N =1024 subcarriers. Furthermore, in this study, the X-transform is utilized to produce a new Alamouti space-time OFDM (ST-OFDM). The proposed ST-X-OFDM scheme reduces the transmitter complexity and achieves important SNR gain over the conventional ST-OFDM systems. The BER performance of the proposed schemes in the presence of solid-state power amplifiers (SSPAs) is also investigated analytically and by simulation. It shows that the X-OFDM is resilient to the SSPAs nonlinear distortion whereas the C-OFDM may lead to BER impairment in the presence of the SSPA. Furthermore, a coding technique to mitigate the sensitivity of the COFDM scheme to the SSPA is also proposed in this study. In this research, mathematical models for the proposed C-OFDM, XOFDM and ST-X-OFDM, which tightly match the simulation results over a diverse range of transmission scenarios and mapping schemes, are also derived. In addition, the BER performance of the proposed COFDM and X-OFDM schemes in the presence of the carrier frequency offset (CFO), with and without frequency synchronization algorithm, are also investigated. The proposed C-OFDM and X-OFDM schemes are more sensitive to the CFO than the conventional schemes. However, when frequency synchronization algorithm is used, both the proposed schemes retain their significant BER improvement in comparison to the conventional schemes.Ministry of Higher Education and Scientific Research (MOHSR), Iraq and to the Iraqi cultural attach- London for supporting me financially during my study in England

Analytical Characterization and Optimum Detection of Nonlinear Multicarrier Schemes

It is widely recognized that multicarrier systems such as orthogonal frequency division multiplexing (OFDM) are suitable for severely time-dispersive channels. However, it is also recognized that multicarrier signals have high envelope fluctuations which make them especially sensitive to nonlinear distortion effects. In fact, it is almost unavoidable to have nonlinear distortion effects in the transmission chain. For this reason, it is essential to have a theoretical, accurate characterization of nonlinearly distorted signals not only to evaluate the corresponding impact of these distortion effects on the system’s performance, but also to develop mechanisms to combat them. One of the goals of this thesis is to address these challenges and involves a theoretical characterization of nonlinearly distorted multicarrier signals in a simple, accurate way. The other goal of this thesis is to study the optimum detection of nonlinearly distorted, multicarrier signals. Conventionally, nonlinear distortion is seen as a noise term that degrades the system’s performance, leading even to irreducible error floors. Even receivers that try to estimate and cancel it have a poor performance, comparatively to the performance associated to a linear transmission, even with perfect cancellation of nonlinear distortion effects. It is shown that the nonlinear distortion should not be considered as a noise term, but instead as something that contains useful information for detection purposes. The adequate receiver to take advantage of this information is the optimum receiver, since it makes a block-by-block detection, allowing us to exploit the nonlinear distortion which is spread along the signal’s band. Although the optimum receiver for nonlinear multicarrier schemes is too complex, due to its necessity to compare the received signal with all possible transmitted sequences, it is important to study its potential performance gains. In this thesis, it is shown that the optimum receiver outperforms the conventional detection, presenting gains not only relatively to conventional receivers that deal with nonlinear multicarrier signals, but also relatively to conventional receivers that deal with linear, multicarrier signals. We also present sub-optimum receivers which are able to approach the performance gains associated to the optimum detection and that can even outperform the conventional linear, multicarrier schemes

Hybrid solutions to instantaneous MIMO blind separation and decoding: narrowband, QAM and square cases

Future wireless communication systems are desired to support high data rates and high quality transmission when considering the growing multimedia applications. Increasing the channel throughput leads to the multiple input and multiple output and blind equalization techniques in recent years. Thereby blind MIMO equalization has attracted a great interest.Both system performance and computational complexities play important roles in real time communications. Reducing the computational load and providing accurate performances are the main challenges in present systems. In this thesis, a hybrid method which can provide an affordable complexity with good performance for Blind Equalization in large constellation MIMO systems is proposed first. Saving computational cost happens both in the signal sep- aration part and in signal detection part. First, based on Quadrature amplitude modulation signal characteristics, an efficient and simple nonlinear function for the Independent Compo- nent Analysis is introduced. Second, using the idea of the sphere decoding, we choose the soft information of channels in a sphere, and overcome the so- called curse of dimensionality of the Expectation Maximization (EM) algorithm and enhance the final results simultaneously. Mathematically, we demonstrate in the digital communication cases, the EM algorithm shows Newton -like convergence.Despite the widespread use of forward -error coding (FEC), most multiple input multiple output (MIMO) blind channel estimation techniques ignore its presence, and instead make the sim- plifying assumption that the transmitted symbols are uncoded. However, FEC induces code structure in the transmitted sequence that can be exploited to improve blind MIMO channel estimates. In final part of this work, we exploit the iterative channel estimation and decoding performance for blind MIMO equalization. Experiments show the improvements achievable by exploiting the existence of coding structures and that it can access the performance of a BCJR equalizer with perfect channel information in a reasonable SNR range. All results are confirmed experimentally for the example of blind equalization in block fading MIMO systems

PAPR and ICI reduction techniques for OFDM based satellite communication systems

Multi-carrier systems such as orthogonal frequency division multiplexing (OFDM) are significantly affected by peak-to-average-power ratio (PAPR). Unfortunately, the high PAPR inherent to OFDM signals envelopes will occasionally drive high power amplifiers (HPAs) to operate in the nonlinear region of their characteristic curve. The nonlinearity of the HPA exhibits amplitude and phase distortions, which cause loss of orthogonality among the subcarriers (SCs), and hence, inter-carrier interference (ICI) is introduced in the transmitted signal. The ICI power is proportional to the amplitude of the signal at the amplifier input and it may cause a considerable bit error rate (BER) degradation. A plethora of research has been devoted to reduce the performance degradation due to the PAPR problem inherent to OFDM systems. Some of the reported techniques such as amplitude clipping have low-complexity; on the other hand, they suffer from various problems such as in-band distortion and out-of-band expansion. Signal companding methods have low-complexity, good distortion and spectral properties; however, they have limited PAPR reduction capabilities. Advanced techniques such as coding, partial transmit sequences (PTS) and selected mapping (SLM) have also been considered for PAPR reduction. Such techniques are efficient and distortionless, nevertheless, their computational complexity is high and requires the transmission of several side information (SI) bits. In this thesis, a new low-complexity scheme is proposed based on the PTS that employs two inverse fast Fourier transforms (IFFTs) and two circulant transform matrices, in order to reduce complexity and improve the system performance. Furthermore, the low-complexity scheme is simplified by omitting one of the circulant transform matrices in order to reduce both the computational complexity and the number of SI bits at the cost of a small reduction in PAPR and BER performance. It is well known that, accurate PAPR estimation requires oversampling of the transmitted signal, which in turn results in increased complexity. More importantly, minimising the PAPR does not necessarily minimise the distortion produced by the nonlinearity of the HPA. Therefore, minimising PAPR does not necessarily imply that the BER will be minimised too. Efficient and less complex schemes for BER reduction of OFDM systems in the presence of nonlinear HPA and/or carrier frequency offset (CFO) are proposed. These proposed techniques are based on predicting the distortion introduced by the nonlinearity of HPA and/or CFO. Subsequently, techniques such as the PTS and SLM are invoked to minimise the distortion and BER. Three distortion metrics are adopted in this thesis: inter-modulation distortion (IMD), peak interference-to-carrier ratio (PICR) and distortion-to-signal power ratio (DSR). Monte Carlo simulations will confirm that the DSR and PICR are more reliable than the PAPR and IMD for selecting the coefficients of the PTS and SLM to minimise the BER. Furthermore, complexity analyses demonstrate that the proposed schemes offer significant complexity reduction when compared to standard PAPR-based methods. A closed form solution for accurate BER for the OFDM signals perturbed by both the HPA nonlinearity and CFO was derived. Good agreement between the simulation results and the theoretical analysis can be obtained for different HPA parameters and CFOs. Finally, efficient approaches to reduce the impact of nonlinear power amplifiers with respect to the BER of OFDM systems are proposed. These are approaches based on: the well-established PAPR schemes, a power amplifier model and a simple single point cross correlator. The optimum phase sequence within the proposed approaches is selected by maximising the correlation between the input and output of the power amplifier model. Simulation results have confirmed that the BER using the proposed approaches is almost identical to the DSR, while the complexity is reduced significantly for particular system configurations.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Space-time-frequency block codes for MIMO-OFDM in next generation wireless systems

In this thesis the use of space-frequency block codes (SFBC) and space-time-frequency block codes (STFBC) in wireless systems are investigated. A variety of SFBC and STFBC schemes are proposed for particular propagation scenarios and system settings where each has its own advantages and disadvantages. The objective is to pro-pose coding strategies with improved flexibility, feasibility and spectral efficiency,and reduce the decoding complexity in an MIMO-OFDM system. Firstly an efficient SFBC with improved system performance is proposed for MIMO-OFDM systems. The proposed SFBC incorporates the concept of matched rotation precoding (MRP) to achieve full transmit diversity and optimal system performance foran arbitrary numberoftransmitantennas,subcarrierinterval andsubcarriergrouping. The MRP is proposed to exploit the inherent rotation and repetition properties of SFBC, arising from the channel power delay profile, in order to fully capture both space and frequency diversity of SFBC in a MIMO-OFDM system. It is able to relax restrictions on subcarrier interval and subcarrier grouping, making it ideal for adaptive/time-varying systems or multiuser systems. The SFBC without an optimization process is unstable in terms of achievable system performance and diversity order, and also risks diversity loss within a specific propagation scenario. Such loss or risk is prominent while wireless propagation channel has a limited number of dominant paths, e.g. relatively close to transmitters or relatively flat topography. Hence in orderto improve the feasibility of SFBC in dynamic scenarios, the lower bound of the coding gain for MRP is derived. The SFBC with MRP is proposed for more practical scenarios when only partial channel power delay profile information is known at the transmit end, for example the wireless channel has dominant propagation paths. The proposed rate one MRP has a relatively simple optimization process that can be transformed into an explicit diagram and hence an optimal result can be derived intuitively without calculations. Next, a multi-rate transmission strategy is proposed for both SFBCand STFBC to balance the system performance and transmission rate. A variety of rate adaptive coding matrices are obtained by a simple truncation of the coding matrix, or by parameter optimization for coding matrices for a given transmission rate and constellation. Pro-posed strategy can easily and gradually adjust the achievable diversity order. As a result it is capable of achieving a relatively smooth balance between system performance and transmission rate in both SFBC and STFBC, without a significant change of coding structure or constellation size. Such tradeoff would be useful to maintain stable Quality of Service (QoS) for users by providing more scalability of achievable performance in a time-varying channel. Finally the decoding procedure of space-time block code (STBC), SFBCand STFBC is discussed. The decoding of all existing STBC/SFBC/STFBC is unified at first, in order to show a concise procedure and make fair comparisons. Then maximum likelihood decoding (MLD) and arbitrary sphere decoding (SD) can be adopted. To reduce the complexity of decoding further, a novel decoding method called compensation de-coding (CD) is presented for a given space-time-frequency coding scheme. By taking advantage of the simplicity of zero-forcing decoding (ZFD) we are able to calculate a compensation vector for the output of ZFD. After modification by utilizing the com-pensation vector, the BER performance can be improved significantly. The decoding procedure is relatively simple and is independent of the constellation size. The per-formance of the proposed decoding method is close to maximum-likelihood decoding for low to medium SNR. A low complexity detection scheme, classifier based decoding (CBD), is further proposed for MIMO systems incorporating spatial multiplexing. The CBD is a hybrid of an equalizer-based technique and an algorithmic search stage. Based on an error matrix and its probability density functions for different classes of error, a particular search region is selected for the algorithmic stage. As the probability of occurrence of error classes with larger search regions is small, overall complexity of the proposed technique remains low, whilst providing a significant improvement in the bit error rate performance